1. Field of the Invention
This invention is directed to anodes in the form of a grid for use in cathodic protection systems.
2. Description of Related Prior Art
Cathodic protection of metal structures, or of metal containing structures, in order to inhibit or prevent corrosion of the metal in the structure is well known by use of impressed current cathodic protection systems. In such systems, counter electrodes and the metal of the structure are connected to a source of direct current. In operation the metal of the structure, such as a steel reinforcement for a concrete structure, is cathodically polarized. The steel reinforcement becomes cathodically polarized being spaced from the anodically polarized electrode and is inhibited against corrosion. While cathodic protection is well known for metal or metal containing structures such as in the protection of offshore steel drilling platforms, oil wells, fuel pipes submerged beneath the sea, and in the protection of the hulls of ships, a particularly difficult problem is presented by the corrosion of steel reinforcement bars in steel-reinforced concrete structures. Most Portland cement concrete is porous and allows the passage of oxygen and aqueous electrolytes. Salt solutions which remain in the concrete as a consequence of the use of calcium chloride to lower the freezing point of uncured concrete or snow or ice melting salt solutions which penetrate the concrete structure from the environment can cause more rapid corrosion of steel reinforcing elements in the concrete. For example, concrete structures which are exposed to the ocean and concrete structures in bridges, parking garages, and roadways which are exposed to water containing salt used for deicing purposes are weakened rapidly as the steel reinforcing elements corrode. This is because such elements when corroded create local pressure on the surrounding concrete structure which brings about cracking and eventual spalling of the concrete.
Impressed current cathodic protection systems are well known for the protection of reinforced concrete structures such as buildings and in road construction, and, particularly, in the fabrication of supports, pillars, crossbeams, and road decks for bridges. Over the years, increasing amounts of common salt, sodium chloride, have been used during the winter months to prevent ice formation on roads and bridges. The melted snow or ice and sodium chloride in aqueous solution tend to seep into the reinforced concrete structure. In the presence of chloride ion the reinforcing steel rebars are corroded at an accelerated rate such that the resultant corrosion products formed by the oxidation reaction occupy a greater volume than the space occupied by the reinforcing bars prior to oxidation. Eventually an increased local pressure is created which brings about cracking of the concrete and eventual spalling of the concrete covering the reinforcing members so as to expose the reinforcing members directly to the atmosphere. The use of a valve metal without an electrocatalytically active coating thereon as an anode in a cathodic protection system is unexpected in view of the belief among those skilled in the art that a titanium anode or an alloy of titanium possessing properties similar to titanium cannot be used in an electrolytic process as the surface of the titanium would oxidize when anodically polarized and the titanium or alloys thereof would soon cease to function as an anode.
For instance, in U.S. Pat. No. 5,334,293, electrocatalytically coated anodes of titanium or an alloy of titanium are disclosed for use in an electrolytic cell, particularly, for use as an anode in an electrolytic cell in which chlorine is evolved at the anode. The coating utilized usually includes a metal of the platinum group, oxides of metals of the platinum group, or mixtures of one or more metals such as one or more oxides or mixtures or solid solutions of one or more oxides of a platinum group metal and a tin oxide or one or more oxides of a valve metal such as titanium. Similar electrocatalytically coated titanium electrodes are disclosed in U.S. Pat. No. 3,632,498; U.S. Pat. No. 5,354,444; and U.S. Pat. No. 5,324,407.
Known methods of introducing an anode into existing concrete structures may involve insertion of an anode into a slot cut into the concrete. After application of the anode a cap of grout is applied to backfill the slot. Representative anodes for cathodic protection of steel reinforced concrete structures are disclosed in U.S. Pat. No. 5,062,934 to Mussinelli in which a grid electrode comprised of a plurality of valve metal strips having voids are disclosed. Another type of anode strip for cathodic protection of steel reinforced concrete structures is disclosed in Canadian 2,078,616 to Bushman in which mesh anodes are disclosed consisting of an electrocatalytically coated valve metal which is embedded in a reinforced concrete structure so as to function as the anode in a cathodic protection system. In U.S. Pat. No. 5,031,290 a process is disclosed for the production of an open metal mesh having a coating of an electrocatalytically active material formed by fitting a sheet and stretching the coated sheet to expand the sheet and form an open mesh. In U.S. Pat. No. 4,401,530 to Clere, a three dimensional electrode having substantially coplanar, substantially flat portions, and ribbon-like curved portions is disclosed for use as a dimensionally stable anode in the production of chlorine and caustic soda. The ribbon-like portions of the anode are symmetrical and alternate in rows above and below the flat portions of the anode.
In U.S. Pat. No. 3,929,607 to Krause, an anode assembly for an electrolytic cell is disclosed comprising a film-forming metal foraminate structure comprising a plurality of longitudinal members spaced with their longitudinal axis parallel to one another and carrying on at least part of their surface an electrocatalytically active coating. Each longitudinal member comprises a channel blade member constituted by a pair of parallel blades having one or more bridge portions connected to the current lead-in means.
It is known from U.S. Pat. No. 5,334,293 that a titanium anode cannot be used in an electrolytic cell, particularly in an electrolytic cell in which during operation of the cell chlorine is evolved at the anode. Such an anode cannot be used in this electrolytic cell as the surface of the titanium anode would oxidize when anodically polarized and the titanium would soon cease to function as an anode. Coatings comprising ruthenium oxide are disclosed as useful on a titanium substrate to obtain an electrode having a commercially useful lifetime.
Bockris et al. in Modem Electrochemistry, volume 2, pages 1315-1321, Plenum Press, explains the transformation of a metal surface from a corroding and unstable surface to a passive and stable surface as being facilitated by increasing the electrical potential in the positive direction on the metal. As the potential is increased, the current initially increases, reaching a maximum value and then starts sharply to decrease to a negligible value. The point at which the current sharply decreases is referred to as passivation and the potential at which this occurs is termed the passivation potential.
In the prior art, electrodes particularly for use in cathodic protection systems require electrocatalytic coatings on valve metals which are subject to passivation in order to overcome the tendency of such metals to passivate and cease to function as electrodes. Such coatings are described in U.S. Pat. No. 3,632,498 as consisting essentially of at least one oxide of a film-forming metal and a nonfilm-forming conductor the two being in a mixed crystal form and covering at least two percent of the active surface of the electrode base metal. Similarly, electrodes made utilizing a valve metal substrate are disclosed as requiring one or more layers of a coating containing platinum as disclosed in U.S. Pat. No. 5,290,415 and U.S. Pat. No. 5,395,500.
An anode useful in a cathodic protection system to protect the reinforcing steel bars in a concrete structure can consist of a porous titanium oxide, TiOx where xe2x80x9cxxe2x80x9d is in the range 1.67 to 1.95, as disclosed in European patent application 186 334 or where xe2x80x9cxxe2x80x9d is in the range 1.55 to 1.95, as disclosed in U.S. Pat. No. 4,422,917. Other porous materials are disclosed in 186 334 as substitutes for the porous titanium oxide such as graphite, porous magnetite, porous high silicon iron or porous sintered zinc, aluminum or magnesium sheet.
In U.S. Pat. No. 4,319,977, an electrode formed of thin sheets of titanium is disclosed as useful in an electrometallurgical cell. In addition to a metal such as titanium, electrodes consisting essentially of tantalum, niobium, or zirconium are disclosed as useful in the British patent no. 951,766 cited in this United States patent. As described in ""977, the titanium electrode is utilized as an anode in a method of electrolytically producing manganese dioxide by immersing the electrode in a solution of manganese sulphate and sulfuric acid and electrolytically depositing the manganese dioxide onto the electrode. Periodically, the manganese dioxide is removed from the electrode.
Expanded mesh anode structures having an electrocatalytic surface which are disclosed as useful for cathodic protection of steel reinforced concrete are disclosed in U.S. Pat. No. 5,421,968, U.S. Pat. No. 5,423,961, and U.S. Pat. No. 5,451,307. These mesh anode structures have 500 to 2000 nodes per square meter formed at metal strand intersections in the mesh and can be supplied in roll form. Upon application to a concrete surface in order to present corrosion of steel reinforcing structures therein, the expanded metal mesh is connected to a current distribution member such as by welding.
A grid electrode is disclosed for use in cathodic protection of steel reinforced concrete structures and a method of forming a grid electrode are disclosed, respectively, in U.S. Pat. No. 5,062,934 and U.S. Pat. No. 5,104,502. The metal members forming the grid electrode comprise a plurality of expanded valve metal strips with voids therein, at least 2000 nodes per square meter formed by intersecting strands of expanded metal, and an electrocatalytic surface thereon. The valve metal strips forming the electrode grid are welded together to form the grid. In use, a current distribution member is also connected at intervals to the electrode grid.
Disclosed are novel valve metal grid electrodes for operation at either high or low current density, particularly, as grid anodes in a cathodic protection system in which iron or steel rods are embedded in a concrete structure or as grid anodes for the cathodic protection of steel pipelines placed in sea water, saline muds, or in the ground. The steel rods or pipelines are protected against corrosion by connecting the novel valve metal grid anodes and the iron or steel pipelines or reinforcing rods in the concrete structure to an electrical circuit and impressing a current sufficient to cause the iron or steel material to act as a cathode in the circuit. The valve metal anode strips which are spaced apart to form the grid electrode can be porous or non-porous, coated with an electrocatalytically active metal or non-coated, and of any shape, for instance, expanded metals, slit and deformed metal strips, rods, tubes, etc.